The minituarization and enhanced function of electronic equipment are increasingly in demand. Semiconductors with a high integration density, have been rapidly developed to perform various functions, operate at high speed and produce high output. As a result, the amount of heat generated from semiconductors is increased. Therefore, semiconductor substrates must be composed of a material having high thermal conductivity. The material conventionally used is an aluminum oxide substrate, however, aluminum oxide has a low thermal conductivity of about 20 w/m.multidot.k. Furthermore, since the aluminum oxide substrate has a thermal expansion coefficient larger than that of silicon, it cannot be firmly joined with silicon semiconductors. Presently, aluminum nitride, which has a thermal conductivity larger than that of aluminum oxide and a thermal expansion coefficient close to that of silicon, has become a noteworthy material as a base material for semiconductor substrates.
Furthermore, since a very large-scale integrated circuit is operated at high speeds or frequencies, to effectively eliminate electric noise, capacitors must be provided on the semiconductor substrate. If chip capacitors are provided outside the semiconductor substrate, however, the inductance in the wiring is unfavorably increased. Alternatively, capacitors can be provided inside the semiconductor substrate. The semiconductor substrate is normally prepared by collectively sintering a ceramic layer and a wiring conductor. To obtain a specified capacitance by providing capacitors inside the normal substrate, dielectric layer sheets or paste having the same composition as that of the ceramic layer need to be laminated together with conductive layers, thereby forming a multilayered substrate. Consequently, when capacitors are provided inside the semiconductor substrate, the structure of the semiconductor substrate becomes complicated, thereby raising manufacturing costs.
To simplify the structure of the substrate, the dielectric layer is made thin to increase the capacitance, and the number of laminate layers is reduced. If the dielectric layer is thin, however, pin holes are easily formed and, as a result, an electric short can easily developed. Additionally, to attain good workability, the dielectric layer must be at least 10 .mu.m thick, thereby restricting the resulting capacitance. Alternatively, the dielectric layer can be made larger in area, to obtain the specified capacitance. This is, however, undesirable due to the restrictions in the substrate design.